1. Field of the Invention
This invention relates to an electric motor having a permanent magnet in a rotor core, such as Brushless DC motor or the like and, more specifically, to a permanent magnet rotor type electric motor in which a reluctance torque is utilized to improve efficiency thereof.
2. Description of the Related Art
In an electric motor such as Brushless DC motor, permanent magnets are embedded in a core of an inner rotor thereof, of which conventional examples are shown in FIG. 23 and FIG. 24. Incidentally, each drawing is a plane view of the inside of the electric motor shown from a plane perpendicular to the rotation axis thereof.
In the conventional example shown in FIG. 23, a rotor core 2 is disposed in a stator core 1 having, for example, 24 slots in which a field magnet rotates. The number of poles of the above electric motor is four, so that four permanent magnets 3 are arranged in the rotor core in accordance with the number of poles.
Each permanent magnet 3 is formed into a band plate shape of rectangular cross-section, and each pair of permanent magnets 3 as the south poles and the north poles is arranged across from each other along a direction perpendicular to a diameter line of the rotor core 2 on the outer circumference side of the rotor core 2. Each permanent magnet 3 is embedded in the rotor core 2 in a direction orthogonal to paper drawn with FIG. 23.
Between the two permanent magnets 3, a hole 4 as flux barrier is formed for avoiding short-circuiting and leaking the magnetic flux occurring between the adjacent permanent magnets. In this case, the hole 4 is represented as a triangle-shaped hole and located at each end of each permanent magnet 3. In the central portion of the rotor core 2, a center hole 5 is opened to pass a rotation shaft (not shown) therethrough.
In this point, when the magnetic flux distribution in a gap portion (between teeth of the stator core 1 and the permanent magnets 3) caused by each permanent magnet 3 is in a sine wave state, torque T of the electric motor is given as T=Pn{.PHI.a.multidot.Ia.multidot.cos .beta.-0.5(Ld-Lq).multidot.Ia.sup.2 .multidot.sin 2.beta.}.
It should be mentioned that .PHI.a is an armature flux-linkage caused by the permanent magnet 3 on the d and q coordinate axes, Ld and Lq are the d-axis inductance and the q-axis inductance respectively, Ia is amplitude of an armature current on the d and q coordinate axes, .beta. is a lead angle of the armature current from the q axis on the d and q coordinate axes, and Pn is a pole-logarithm.
In the above expression, the first term expresses a magnet torque generated by the permanent magnets 3 and the second term expresses a reluctance torque generated by the difference between the d-axis inductance and the q-axis inductance. Refer to a treatise published in T. IEE Japan, vol. 117-D, No. 8. 1997 for further detail.
In the rotor core 2 shown in FIG. 24 as another conventional example, a permanent magnet 6 of arc-shaped cross-section is used, of which torque T is also given by the aforementioned expression.
However, in the conventional examples, the permanent magnet 3 or 6 having low magnetic permeability is arranged on a magnetic circuit of the d-axis and nearly perpendicular thereto, therefore the inductance Ld on the d-axis is originally small. On the other hand, the comparatively large permanent magnet 3 or 6 is embedded in and along a magnetic circuit of the q-axis, therefore, the inductance Lq on the q-axis is larger than the inductance Ld on the d-axis, but the inductance Lq on the q-axis is not very different from the inductance Ld on the d-axis.
As described thus far, there are disadvantages that the value of the difference between inductances (Ld-Lq) as the parameter in the aforementioned mathematical expression for calculating a torque is small, and the reluctance torque is small.